This invention relates to echo cancellation in telephones and, in particular, to operating a telephone in full duplex operation in the presence of an open acoustic path. As used herein, “telephone” includes cellular telephones.
There are two kinds of echoes in telephones, an acoustic echo from the path between an earphone or a speaker and a microphone and a line echo generated in the switched network for routing a call between stations. Acoustic echo is typically not much of a problem in a wired telephone with a handset. For speaker phones and cell phones, acoustic feedback is much more of a problem. In a speaker phone, a room and its contents becomes part of the audio system and provide an acoustic path from speaker to microphone. In a cell phones, the case provides an acoustic path from speaker to microphone.
There are several potential sources for line echoes. Hybrid devices (two-wire to four-wire converters) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of line echo.
Echo is an instability in a system depending upon gain and delay. An echo is perceived if a delay is greater than approximately twenty milliseconds at normal listening levels. At higher gains, shorter delays can be perceived as a ringing tone. The distance that a signal travels causes a minimum delay. Digital calling apparatus further delays a signal in the digitizing process and in the batch (packet) mode that signals are often handled. Using a satellite relay can add considerably to the delay; a minimum of 250 milliseconds each way. Digital packet transmission through a satellite can produce a delay in excess of 600 milliseconds. Modern network equipment is incapable of handling a delay longer than about 100 milliseconds. Acoustic delays, such as reverberations in a room, can be much longer, up to 1,500 milliseconds.
In a constantly changing environment, such as a telephones, both electronic delays and acoustic delays can change during a call. In the prior art, the settings for an echo cancelling circuit are not changed during a call, largely due to a long convergence time in the circuitry for finding and cancelling an echo. Changing settings during a call would cause noticeable distortion in the sound, somewhat like listening to a recording on magnetic tape when the tape is deformed.
Apparatus for removing or minimizing echoes include echo suppressers, echo cancellers, and adaptive filters; see Digital Signal Processing in Telecommunications by Kishan Shenoi, Prentice-Hall, 1995, Chapter 6 (pages 334-385). “Suppression” is attenuation. Echo cancelling involves subtracting a local replica of the echo from the signal to eliminate an echo. The local replica is created by filtering the signal with an adaptive filter. The adaptive filter models either the near-end (speaker to microphone) or the far end (line out to line in) transfer function, which is assumed to be linear and time invariant; Shenoi, pg. 348. Unfortunately, the assumption is somewhat optimistic.
U.S. Pat. No. 6,282,176 (Hemkumar) and U.S. Pat. No. 6,212,273 (Hemkumar et al.) also discuss the problem of a non-linear echo path. It is proposed to avoid clipping by using automatic gain control. Poor speaker quality is noted as a problem but is not quantified. The data sheet for Speakerphone Chip CS6420, supplied by patentee, Cirrus Logic, Inc., quantifies quality as a speaker having less than two percent total harmonic distortion. Unfortunately, such speakers are expensive and not likely to be found in a speaker phone or any other communication device. When non-linearities are encountered, the system must go half duplex to avoid divergence and distortion. The noticeable drop in signal amplitude to one party is disconcerting.
Filtering a voice signal to eliminate either or both kinds of echo is a particular form of attenuation known in the art. Devices known as complementary comb filters eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone. Matching, rather than complementary, comb filters can be used in the line out and line in channels of a telephone if one also uses a frequency shift; see U.S. Pat. No. 5,386,465 (Addeo et al.). Frequency shifting is undesirable because of the adverse effect on the quality of the voice signal.
Even with well designed band pass filters, a comb filter necessarily reduces the power and spectral content of speech. For example, an amplitude peak may happen to fall within the stop band of a comb filter, substantially changing the sound characteristic of a person's voice. When fricatives fall within a stop band, intelligibility can be significantly reduced. Amplification is not a cure if the filters do not match the spectral response of an person's voice.
In other applications, e.g. automotive cellular telephones, certain sounds are noises characteristic of the vehicle or environment rather than the driver and it would be desirable to have a stop band match the dominant frequency of the noise. Again, comb filters of the prior art cannot remove such noise except by chance.
The tools primarily used in the prior art for removing echoes are an adaptive echo canceller and residual echo suppression (e.g. attenuation and center clipping). The Siemens/Infineon PSB2170 Acoustic Echo Cancelling chip goes a step further to include a Wiener filter in the transmit channel to achieve additional attenuation (−30 dB vs. −20 dB without the filter). A problem with this approach is that adding a filter in series also adds delay to a channel. In the case of the Wiener filter, the data sheet for the PSB2170 indicates a delay of 38-43 ms, as opposed to a delay of less than 1 ms. without the filter. The data sheet also discloses that the Wiener filter is by-passed when speech is detected, as it must be because any delay longer than about 20 ms. is perceptible.
An adaptive echo canceller can use a variety of filters because the canceller is in parallel with the delay path, not in series with it. Up to a point, delay is helpful in an adaptive echo canceller. The data sheet for the PSB2170 chip discloses (page 32) using sub-band filtering in the adaptive echo canceller portion of the circuit.
While the prior art is replete with improvements to either an adaptive echo canceller or a residual echo suppresser, or both, the fact remains that, under typical conditions, a telephone call on a speaker phone that has full duplex capability and the ability to select either mode operates in full duplex less than half the time. This does not include speaker phones that have full duplex capability and are set to half duplex by an installer and does not include the host of half duplex speakerphones in use. Having full duplex capability and not being able to use it is simply a waste of money.
In view of the foregoing, it is therefore an object of the invention to provide improved apparatus for cancelling acoustic echoes and line echoes in telephones while providing full duplex operation most (>90%) of the time during typical operating conditions.
Another object of the invention is to provide an echo cancelling circuit that is less sensitive to non-linearities in the echo path than circuits of the prior art.
A further object of the invention is to provide an echo cancelling circuit that separately and selectively applies suppression, sub-band filtering, and adaptive echo cancelling to a signal to provide as much as 60 dB suppression of an echo.
Another object of this invention is to provide a minimum of 40 dB of echo suppression in a telephone.